1. Field of Use
The present invention generally concerns the reduction of thermal response in superconducting quantum interference device (SQUID) sensors. The present invention particularly concerns the substitution of a non-superconducting for a superconducting modulation coil in SQUIDs (nominally of the "hybrid" type) for the purposes of reducing the long-term, or d.c., component of thermal response; and the further method of temporarily emplacing an alternating current waveform upon such modulation coil for the purpose of initializing the SQUID to thereafter exhibit a lower total thermal response in both d.c. and short-term, or a.c., components.
2. Background of the Prior Art
It has long been known that the output of SQUID sensors of diverse types of construction, and both radio frequency (r.f.) and direct current (d.c.) in operation, do suffer changes, or drifts, with changes in the temperature of the cryogenic media (normally liquid helium) within which such SQUID sensors are immersed. Changes in the pressure within the dewar containing the cryogen within which the SQUID is immersed may also result in changes in the output of the SQUID sensor. These changes in SQUID output caused by temperature changes, or by pressure changes, or by temperature and by pressure changes are not desirable. In the use of SQUIDs for mobile sensor systems, such as in airborne superconducting gradiometer magnetometer sensor systems, those variations in SQUID sensor output associated with motion and with irreducible motion induced temperature flucuations have been the predominant source of sensor system noise, such noise as has typically been in excess of the desired sensitivity of the sensor system for certain practical uses, such as the detection of large ferromagnetic masses.
A large variety of mechanisms has been proposed to explain the significant thermal sensitivity problem in superconducting SQUID-based sensor systems. The mechanisms include thermal expansion, thermal electric effects, thermal dependence of the susceptibility of magnetic impurities, temperature dependence of junction critical currents, thermally induced changes in the r.f. pick-up coil, the presence of materials with superconducting transitions near the operating temperature, the temperature dependence of the penetration depth of superconducting materials, and the temperature dependence of the superconducting coherence length. A coherent, comprehensive, comparative assessment of these hypothetical contributions to the thermal response of SQUID sensors has not been undertaken, but many disparate observations and theories abound. By action of theoretical analysis undertaken in conjunct with the present invention, and by empirical observation of the results of the apparatus and method of the present invention in reducing the thermal response of SQUID sensors, it is hypothesized that the temperature dependence of superconducting coherence length is the most significant mechanism contributing to the thermal response of SQUID sensors, as well as being that mechanism which is most effectively dealt with in its respective long-term and short-term components by the respective apparatus and method of the present invention. Therefore the prior art dealing with the temperature dependence of the superconducting coherence length is most pertinent to the theory of the present invention as such invention is currently theoretically perceived to work.
The temperature dependence of the superconducting coherence length in SQUIDs produces a temperature dependence of the magnetic field distribution caused by a phenomena known as trapped flux, or fluxoids. A fluxoid is a discontinuity, or wormhole, in the magnetic field within a superconducting solid. A fluxoid represents a small region, of radius equal to the superconducting coherence length, of superconductor which remains non-superconducting because the magnetic field contained therein such small region is greater than the superconducting critical magnetic field. It is well known in the prior art that a superconductor cannot remain superconducting in the presence of a magnetic field strength greater than the superconducting critical magnetic field strength. The origin of this high field strength in the small region of the fluxoid is hypothesized to occur from the existence of a pinning site, being a stressed area of lower critical temperature T.sub.C than the surrounding portions of superconducting material which have already turned superconducting, existing within a superconducting solid during the cool-down of such superconducting solid. The region containing magnetic flux, completely encapsulated and surrounded by already superconducting material, continues to shrink in size as the temperature is reduced, the total flux within this region remaining, however, constant. Finally, at the lowest temperatures the size of the non-superconducting region, remaining non-superconducting because of the magnetic field strength contained therein, does equal a domain of radius equal to the coherence length: a trapped fluxoid. A prior art references dealing with trapped flux is the book by R. P. Heubeuer, Magnetic Flux Structures in Superconductors, (Springer-Verlag, Berlin 1979).